I've been custom building computers for people for over 25 years. Determining how much RAM a system needs is simple...but, at the same time, most people don't understand how to actually make that determination. First, understand the difference between "minimum" and "recommended". The best way of thinking about the "recommended" specs is if you were to be running ONLY that app. In that sense, you need to have the "recommended" amount of RAM for that app, plus the "recommended" amount for the OS. However, when buying a new computer, this is NOT how you determine how much RAM you need.
When buying a new computer, in order to figure out the absolute MINIMUM amount needed, start with the "minimum" requirements of the operating system. Next, add to that the "minimum" requirements of ALL applications you could be running at the same time. That doesn't mean add together the minimum RAM requirements of all apps you plan on installing...just the minimum requirements of all the apps you would ever be running at the same time.
Where people tend to 'goof' this up is that they forget about the apps running in the background, such as security suites, graphics drivers, printer drivers, etc. Truthfully, these don't add up to a huge amount...but, they do still add up. Unfortunately, these "minimum requirements" keep increasing, which is why the minimum amount of RAM is constantly increasing. It used to be that 1GB was the "minimum", and 2GB "recommended" (go back even further, and the amounts were even less). Just 10 years ago, the amounts were 3-4GB & 6-8GB. We've now reached a point in time where the absolute "minimum" is 8GB, and a more realistic minimum is 12GB.
My desktop, which is used for everything from email & YouTube, to gaming, video editing & Photoshop, has 32GB. If you're plan is to only use the computer for 3D printing (and maybe email), 8GB should be enough. However, if you expect to be using the computer for multiple purposes, I would highly recommend nothing less than 16GB.
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